1. Field of the Invention
The present invention relates to an apparatus for programming threshold voltage of a non-volatile memory cell and a method therefor, and more particularly to an apparatus for programming threshold voltage of a non-volatile memory cell by which a threshold voltage of a memory cell to be programmed is precisely controlled by a voltage of a word line so as to program a plurality of threshold voltages in a constant range of the threshold voltage, the programming time can be reduced by an automatic reference programming method, and consumption power in programming can be also reduced, and a method therefor.
2. Description of the Conventional Art
General methods for programming a threshold voltage of a non-volatile memory cell refer to a technique that a program voltage pulse is applied for a predetermined time and then the programmed state of the threshold voltage of the memory cell is inspected. In this method, a distribution of the threshold voltage is determined by the time interval during which program pulses are applied, and the programming time is determined by the time during which the programming and inspecting are repeatedly done.
Thus, in order to prevent the repetition of reference to the program, an automatic reference programming method was proposed, which, however, is merely applicable to single-level programs, failing defining definitive relation between the threshold voltage of the memory cell to be programmed and a reference current or reference voltage determining the termination of the program.
FIG. 1 is a circuit diagram of an apparatus for programming a threshold voltage of a flash memory cell out of non-volatile memory cells adapting a conventional automatic reference programming method. As shown in this drawing, the apparatus for programming the threshold voltage of a flash memory cell includes a cell array 1 having a plurality of flash memory cell consisting of a floating gate, a gate thereof being connected to a word line, a drain thereof being connected to a bit line, and a source thereof being connected to a ground voltage VSS; a switching unit 2 for selectively transmitting a program voltage VPRO according to a control signal PROC; a bit line selector 3 for selectively connecting the program voltage VPRO transmitted from the switching unit 2 to the bit line according to a first to Mth bit line selecting signals; a word line selector 4 for selecting a specific word line according to a first to Nth word line select signals; a resistor 5 for converting a cell current ICELL flowing at the bit line to a voltage; a comparator 6 for outputting a result SOUT upon comparison of the converted voltage by the resistor 5 to a reference voltage VREF; and a controller 7 for receiving a state data SD and outputting a program control signal PROC for controlling a program process according to the output of the comparator 6 to the switching unit 2.
The operation of the conventional apparatus for programming the threshold voltage of flash memory cell constructed as described above will now be explained.
First, the word line selector 4 selects a word line connected to a memory cell desired to be programmed according to the first to Nth word line select signals WLSO-WLS(N-1), and applies a predetermined voltage to the selected word line.
At this time, the bit line selector 3 selects a bit line connected to the memory cell desired to be programmed according to the first to Mth bit line select signals BLS0-BLS(M-1).
Accordingly, the memory cell selected by the bit line selector 3 and the word line selector 4 is processed to be programmed.
During the programming, the controller 7 outputs a program control signal PROC to the switching unit 2, and the switching unit 2 applies the program voltage VPRO to the selected bit line of the bit line selector 3 according to the program control signal PROC.
Accordingly, the program voltage VPRO is applied to the memory cell connected to the selected bit line and the word line, to thereby program the memory cell. At this time, a threshold voltage of the memory cell being programmed is increased, so that a cell current ICELL flowing at the bit line connected to the pertinent memory cell is decreased.
In the meantime, the resistor 5 converts the cell current ICELL flowing at the bit line of the selected memory cell to a voltage and applies it to the comparator 6. Then, the comparator 6 compares the voltage converted by the resistor 5 with the initially predetermined reference voltage VREF and outputs its result to the controller 7.
In order to end the program by the output SOUT of the comparator 6 at a specific time when the voltage converted by the resistor 50 becomes greater than the reference voltage VREF, the controller 7 outputs the program control signal PROC to the switching unit 2 so as to block the program voltage VPRO.
However, the conventional apparatus for programming the threshold voltage of the flash memory cell has disadvantages in that if the threshold voltage of the memory cell is initially dropped down, and thus, the current overflows, the cell current ICELL flowing at the bit line drops down, causing the programming speed to be reduced. In addition, after the programming is finished, since the relation between the threshold voltage of the programmed cell and the reference voltage is indistinct, the threshold voltage is to be determined relying on experience bases.